Thermotropic liquid crystals are generally crystalline compounds with significant anisotropy in shape. That is, at the molecular level, they are characterized by a rod-like or disc like structure. When heated they typically melt in a stepwise manner, exhibiting one or more thermal transitions from a crystal to a final isotropic phase. The intermediate phases, known as mesophases, can include several types of smectic phases wherein the molecules are generally confined to layers; and a nematic phase wherein the molecules are aligned parallel to one another with no long range positional order. The liquid crystal phase can be achieved in a heating cycle, or can be arrived at in cooling from an isotropic phase. A comprehensive description of the structure of liquid crystals in general, and twisted nematic liquid crystals in particular, is given in “The Physics of Liquid Crystals,” P. G. de Gennes and J. Prost, Oxford University Press, 1995.
An important variant of the nematic phase is one wherein a chiral moiety is present therein, referred to as a twisted nematic, chiral nematic, or cholesteric phase. In this case, the molecules are parallel to each other as in the nematic phase, but the director of molecules (the average direction of the rodlike molecules) changes direction through the thickness of a layer to provide a helical packing of the nematic molecules. The pitch of the helix is perpendicular to the long axes of the molecules. This helical packing of anisotropic molecules leads to important and characteristic optical properties of twisted nematic phases including circular dichroism, a high degree of rotary power; and the selective reflection of light, including ultraviolet, visible, and near-IR light. Reflection in the visible region leads to brilliantly colored layers. The sense of the helix can either be right-handed or left-handed, and the rotational sense is an important characteristic of the material. The chiral moiety either may be present in the liquid crystalline molecule itself, for instance, as in a cholesteryl ester, or can be added to the nematic phase as a dopant, leading to induction of the cholesteric phase. This phenomenon is well documented, as discussed for example in H. Bassler and M. M. Labes, J. Chem. Phys., 52, 631 (1970).
There has been significant effort invested in methods for preparing, by synthesis, polymerization and otherwise, stable polymer layers exhibiting fixed cholesteric optical properties. One approach has been to synthesize monofunctional and/or polyfunctional reactive monomers that exhibit a cholesteric phase upon melting, formulate a low melting liquid crystal composition, and polymerize the liquid crystal composition in its cholesteric phase to provide a polymer network exhibiting stable optical properties of the cholesteric phase. Use of cholesteric monomers alone, as disclosed in U.S. Pat. No. 4,637,896, provided cholesteric layers with the desired optical properties, but the polymer layers possessed relatively weak mechanical properties.
Many efforts have been made to improve the physical properties and thermal stabilities by formulating twisted nematic monomer phases that are capable of crosslinking polymerizations to provide polymer networks. A need remains, however, for polymerizable chiral monomers that have good phase compatibility in polymerizable nematic liquid crystals and high helical twisting power (HTP). High HTP allows the use of smaller amounts of the expensive chiral component to be used in twisted nematic formulations to induce a desired pitch. Good phase compatibility is required to prevent premature crystallization or phase separation of the chiral monomers from the twisted nematic formulation.
Crosslinking chiral monomers with high HTP and good phase compatibility, based on the isosorbide chiral group, are disclosed in U.S. Pat. No. 6,723,395. Chiral monomers with high HTP and good phase compatibility, based on alkaloid monomers are disclosed in U.S. Pat. No. 7,022,259. However, the latter chiral dopants are not capable of being substantially covalently bonded to a polymer network. In some applications these nonpolymerizable monomers may exhibit adequate stability. However, for many optical applications, the cholesteric layer is in contact with another layer. The other layer may be another cholesteric layer or a different material. In this case, stability of the chiral dopant toward migration and/or extraction becomes a significant issue. This is because, as discussed above, the concentration and HTP of the chiral dopant significantly determines the pitch and thus the wavelength of maximum reflection. Thus, a need remains for chiral monomers that have good phase compatibility with polymerizable nematic phases, exhibit high HTP, and exhibit significant stability in polymer networks toward migration.